Flash Memory Device

ABSTRACT

A method of manufacturing a flash memory device is disclosed. The method includes the steps of providing a semiconductor substrate in which a cell region and a select transistor region are defined, etching the semiconductor substrate in the select transistor region so that there is a first step between the cell region and the select transistor region, forming a cell gate in the cell region, and forming a transistor in the select transistor region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 11/758,509 filed Jun. 5,2007, which claims the priority benefit under USC 119 of Korean patentapplication number 10-2006-121518, filed on Dec. 4, 2006, the respectivedisclosures of which are incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates, in general, to flash memory devices and, moreparticularly, to a method of manufacturing a flash memory device thatreduces an undesirable interference phenomenon.

Semiconductor memory devices for storing data can be largely classifiedinto volatile memory devices or nonvolatile memory devices. Volatilememory devices lose their data when the supply of power thereto isstopped, whereas nonvolatile memory devices retain their data in theabsence of power.

Each nonvolatile memory device includes a flash memory device. A unitcell of the flash memory device generally includes an active regiondefined on a specific region of a semiconductor substrate, a tunnelinsulating layer formed on the active region, a floating gate formed onthe tunnel insulating layer, a gate interlayer insulating layer formedon the floating gate, and a control gate electrode formed on the gateinterlayer insulating layer. In particular, the flash memory device hasbeen widely used for MP3 players, digital cameras, memory for computerBIOS storage, mobile phones, portable data storage devices and so on.

The flash memory cell can store data as voltage externally applied tothe control gate electrode is coupled to the floating gate. Accordingly,when storing data for a short period of time and at a low programvoltage, the ratio of voltage applied to the control gate electrode tovoltage applied to the floating gate must be high. The ratio of voltageapplied to the control gate electrode to voltage applied to the floatinggate is called the coupling ratio (“CR”). Furthermore, the couplingratio can be expressed by the ratio of the capacitance of the gateinterlayer insulating layer to the sum of the capacitance of the tunnelinsulating layer and the gate interlayer insulating layer.

In current flash memory manufacturing methods, the space where a unitactive region and a unit field region will be formed narrows as devicesbecome more highly integrated. When a dielectric layer, a control gateand a floating gate are formed within a narrow active space, thedistance between the gates is narrowed, so that the interferencephenomenon becomes increasingly problematic. In particular, in order todevelop a Multi-Level Cell (MLC) in a general NAND flash memory deviceemploying an Advanced Self-Aligned Shallow Trench Isolation (ASA-STI)method, interference charges between the floating gates must be reduced.

A conventional method of manufacturing a flash memory device is brieflydescribed below.

In order to minimize the damage to a semiconductor substrate resultingfrom an ion implant process, a buffer insulating layer is formed on thesemiconductor substrate. The buffer insulating layer is formed from anoxide layer. After the ion implant process is performed, the bufferinsulating layer is removed. After a tunnel insulating layer is formedover the semiconductor substrate, an annealing process is performed. Thetunnel insulating layer is formed by performing a wet or dry oxidizationprocess at a temperature ranging from 750° C. to 850° C. using H₂ or O₂.The annealing process is performed in order to eliminate the trapphenomenon occurring at the interface between the semiconductorsubstrate and the tunnel insulating layer and to increase the datastorage ability of the transistor. The annealing process includes apost-anneal process at a temperature ranging from 750° C. to 1100° C.using N₂, N₂O, or NO.

A first conductive layer for a floating gate is formed on the tunnelinsulating layer. The first conductive layer is formed by in-situimplanting a P or B dopant into a source gas of SiH₄, Si₂H₆, or SiH₂Cl₂.A first hard mask layer having a stack structure of an oxide layer and anitride layer is formed on the first conductive layer. The first hardmask layer, the first conductive layer, the tunnel insulating layer, andthe semiconductor substrate are partially etched by a photo anddevelopment processes, thereby forming trenches and also a floating gateconsisting of the first conductive layer.

A first insulating layer is formed over the semiconductor substrateincluding the trenches so that the trenches are filled. The firstinsulating layer is polished to form isolation layers. The firstinsulating layer is polished using a High Density Plasma (HDP) oxidelayer or Spin on Glass (SOG). In order to control the Effective FieldHeight (EFH) of the isolation layers, a wet etch process (i.e., acleaning process) is performed to partially remove the top surface ofthe isolation layers.

A dielectric layer is formed over the semiconductor substrate includingthe isolation layers and the first conductive layer. An annealingprocess and an oxidization process are carried out at a temperatureranging from 600° C. to 900° C. using O₂ or H₂. A second conductivelayer for a control gate and a second hard mask layer are formed overthe dielectric layer. The second conductive layer has a stackedstructure including a polysilicon layer and a tungsten silicide(WSi_(x)) layer or tungsten (W). The second hard mask layer is formedfrom an oxide layer or a silicon oxynitride (SiON) layer. The secondhard mask layer, the second conductive layer, the dielectric layer andthe first conductive layer are etched through photo and developmentprocesses, thereby forming gates.

Spacers for separating the gates are formed on the sides of the gates.In order to protect a cell gate, a second insulating layer is formedover the semiconductor substrate including the spacers and the gates.The second insulating layer is formed from an oxide layer or a nitridelayer.

A first insulating layer having a source contact plug formed therein isformed on the second insulating layer. A second insulating layer havinga drain contact plug formed therein is formed over the semiconductorsubstrate including the source contact plug and the first insulatinglayer. The source contact plug and the drain contact plug are formedfrom polysilicon or tungsten. In order to store data in the cell gateand transfer external voltage to the source contact plug and the draincontact plug formed so as to read stored data, metal lines are formed onthe second insulating layer.

Today, in a flash memory device, in order to select a specific cellgate, voltage is applied to the SSL and the metal line, and currentflows through the cell gate due to a difference between appliedvoltages. In order for the current to flow through only the cell gate,the select transistor, such as the Source Select Line (SSL) or the drainselect line (DSL), are used. However, an interference phenomenon occursbetween the select transistor and the cell gate that is the nearest tothe select transistor due to voltage applied to turn on/off the selecttransistor. Accordingly, the current properties of the gate around theselect transistor are changed, causing a large amount or small amount ofcurrent to flow. This results in defective cell properties.

SUMMARY OF THE INVENTION

Accordingly, the invention addresses the above problems and discloses amethod of manufacturing a flash memory device that reduces theinterference phenomenon between a select transistor and a cell gate insuch a manner that a cell region and a select transistor region have astep created by partially etching the select transistor region of asemiconductor substrate (i.e., a region where a Source Select Line (SSL)and a Drain Select Line (DSL) are formed).

According to an aspect of the invention, there is provided a method ofmanufacturing a flash memory device, including the steps of providing asemiconductor substrate in which a cell region and a select transistorregion are defined, etching the semiconductor substrate in the selecttransistor region so that there is a first step between the cell regionand the select transistor region, forming a cell gate in the cellregion, and forming a transistor in the select transistor region.

According to another aspect of the invention, there is provided a flashmemory device, including a semiconductor substrate having a step betweena cell region and a select transistor region, a transistor formed in theselect transistor region, and a memory cell formed in the cell region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are cross-sectional views illustrating a method ofmanufacturing a flash memory device according to an embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

Now, a specific embodiment according to the invention is described withreference to the accompanying drawings.

Referring to FIG. 1A, a first insulating layer (not shown) is formed ona semiconductor substrate 100 in order to minimize damage to thesemiconductor substrate resulting from an ion implant process. The firstinsulating layer can be formed at a thickness of 50 Å to 200 Å using anoxide layer or a nitride layer. In order to define a cell region and aperi region, an N-type or P-type impurity is implanted into thesemiconductor substrate 100, and the first insulating layer is thenremoved.

A select transistor region (i.e., a region of the semiconductorsubstrate 100 in which a SSL and a DSL will be formed) is partiallyetched to form first trenches 102. The width of the first trenches 102can be set to 4 to 6 times larger than that of the SSL or the DSL, andthe depth of the first trenches 102 can range from 300 Å to 1000 Å. Whenforming the first trenches 102, the side of the first trenches 102 issloped, not vertical. The angle of the slope can be controlled dependingon the polymer formation method and the corresponding amount of polymerused when etching the semiconductor substrate 100. The side of the firsttrenches 102 is sloped so that it can provide good etching selectivitywhen etching steps are performed subsequent to the deposition of severaladditional layers. The first trenches 102 formed in the selecttransistor region define a step formed between the cell region and theselect transistor region.

Referring to FIG. 1B, a tunnel insulating layer 104 is formed over thesemiconductor substrate 100 including the first trenches 102. In orderto prevent a trap phenomenon from occurring at the interface of thesemiconductor substrate 100 and the tunnel insulating layer 104 and toincrease the data storage ability of the transistor, an annealingprocess is performed. The tunnel insulating layer 104 can be formed byperforming a wet or dry oxidization process at a temperature rangingfrom 750° C. to 850° C. using H₂ or O₂. The annealing process can beperformed as a post annealing process at a temperature ranging from 750°C. to 1100° C. using N₂, N₂O, or NO.

A first conductive layer 106 for a floating gate and a first hard masklayer (not shown) are formed on the tunnel insulating layer 104. Thefirst conductive layer 106 can be formed from a polysilicon layer byin-situ implantation of a Phosphorous(P) or Boron(B) dopant into asource gas, such as SiH₄, Si₂H₆, or SiH₂Cl₂. The first hard mask layercan be formed using a single pad oxide layer, a single nitride layer, ora stacked layer including a pad oxide layer and a nitride layer.

The first hard mask layer, the first conductive layer 106, the tunnelinsulating layer 104 and the semiconductor substrate 100 are partiallyetched through photo and development processes, thereby forming secondtrenches (not shown) for isolation layers (not shown) and also afloating gate pattern (not shown) comprising the first conductive layer106.

A second insulating layer is formed over the semiconductor substrate 100including the second trenches so that the second trenches are filled.The second insulating layer is then polished to form the isolationlayers. The second insulating layer can be formed from an HDP oxidelayer or SOG. In order to control the EFH of the isolation layer, a wetetch process (i.e., a cleaning process) is performed to partially removethe top surface of the isolation layer.

A dielectric layer 108 is formed over the semiconductor substrate 100including the isolation layer and the first conductive layer 106. Anannealing process and an oxidization process are then performed. Theannealing process can be performed at a temperature ranging from 600° C.to 900° C. using O₂ or H₂.

A second conductive layer 110 for a control gate and a second hard masklayer 112 are formed over the dielectric layer 108. The secondconductive layer 110 can have a structure in which a polysilicon layeris stacked with a tungsten silicide (WSi_(x)) layer or a tungsten (W)layer. The second hard mask layer 112 can be formed from an oxide layeror a silicon oxynitride (SiON) layer.

Referring to FIG. 1C, the second hard mask layer 112, the secondconductive layer 110, the dielectric layer 108, the first conductivelayer 106 and the tunnel insulating layer 104 are sequentially etchedthrough photo and development processes, thereby forming gates 114 inwhich the tunnel insulating layer 104, the first conductive layer 106,the dielectric layer 108, the second conductive layer 110 and the secondhard mask layer 112 are stacked.

When cell gates 114 (i.e., the gates 114 in the cell region) are formed,a SSL and a DSL (which have the same stacked structure as the cell gates114) are also formed at the same time in the select transistor region(i.e., the region in which the first trenches 102 are formed). A step isformed between the cell gates 114 in the cell region, and the SSL or theDSL formed in the select transistor region due to the first trenches102.

Referring to FIG. 1D, a re-oxidation process is performed in order toeliminate damage occurring in the etch process for forming the gate 114.In order to separate the gates 114, spacers 116 are formed on the sidesof the gates 114. In order to protect the cell gates 114, a thirdinsulating layer 118 is formed over the semiconductor substrate 100including the spacer 116 and the gates 114. The third insulating layer118 can be formed from an oxide layer or a nitride layer.

Referring to FIG. 1E, a first insulating layer 120 is formed on thethird insulating layer 118. When the first insulating layer 120 isformed, there is a step in the select transistor region. The firstinsulating layer 120 can be polished using a Chemical MechanicalPolishing (CMP) process to remove the step.

The first insulating layer 120 is etched to form a source contact holein a source retion between the SSLs of the select transistor region. Thesource contact hole is filled with a third conductive layer to form asource contact plug 122. The third conductive layer can be formed frompolysilicon or tungsten (W).

Thereafter, a second insulating layer 124 is formed over thesemiconductor substrate 100 including the source contact plug 122 andthe first insulating layer 120. A CMP process is then performed topolish the second insulating layer 124. When the second insulating layer124 is formed, there is a step in the select transistor region. In thiscase, if a photoresist pattern for forming a drain contact hole isformed without first removing the step, the photoresist pattern willalso have a step, preventing the etch process from being properlyperformed. Furthermore, if the etch process is not properly performed,metal material partially remains, thereby degrading the cell properties.

The second insulating layer 124 and the first insulating layer 120 areetched by means of photo and development processes, thereby forming adrain contact hole in a drain region between the DSLs of the selecttransistor region. A fourth conductive layer is then formed over thesemiconductor substrate 100 including the drain contact hole so that thedrain contact hole is filled. The fourth conductive layer can be formedfrom polysilicon or tungsten (W).

A polishing process is then performed until the top surface of thesecond insulating layer 124 is exposed, thereby forming a drain contactplug 126.

As described above, the invention has the following advantages.

First, the cell region and the select transistor region are formed tohave a step by partially etching the select transistor region of asemiconductor substrate (i.e., the region in which the SSL and the DSLare formed). Accordingly, the interference phenomenon between the selecttransistor and the cell gate nearest to the select transistor can beimproved.

Second, by reducing the interference phenomenon, the width between theselect transistor and the cell gate nearest to the select transistor canbe reduced significantly.

Third, by reducing the interference phenomenon, the cell properties canbe prevented from becoming defective since a large amount or a smallamount of voltage flows due to changed current properties of a cell gatenear a select transistor.

Fourth, a retention characteristic of the select transistor and the cellgate nearest to the select transistor can be improved.

Fifth, by improving the interference phenomenon, a programcharacteristic of the cell gate can become uniform.

Although the foregoing description has been made with reference to thevarious embodiments, it is to be understood that changes andmodifications of the disclosure may be made by the skilled artisanwithout departing from the spirit and scope of the disclosure andappended claims.

1. A flash memory device, comprising: a semiconductor substrate having astep between a cell region and a select transistor region; a transistorformed in the select transistor region; and a memory cell formed in thecell region.
 2. The flash memory device of claim 1, wherein a width ofthe select transistor region has 4 to 6 times larger than that of thetransistor.
 3. The flash memory device of claim 1, wherein the step hasa height ranging from about 300 Å to about 1000 Å.
 4. The flash memorydevice of claim 1, wherein a source contact plug and a drain contactplug are formed in the select transistor region.